SEASONAL-VARIATIONS IN AIR-WATER EXCHANGE OF POLYCHLORINATED-BIPHENYLS IN LAKE-SUPERIOR

Instantaneous and seasonal fluxes of polychlorinated biphenyls (PCBs) across the Lake Superior air-water interface were predicted using a modified two-film gas-exchange model. Instantaneous fluxes of SIGMAPCB (sum of 80 congeners) were calculated from air and water samples collected simultaneously during short cruises in 1988,1990, and 1992. Volatilization fluxes dominated in late summer in warmer waters (15-22-degrees-C) while EPCBs were deposited into cool springtime waters (approximately 2-degrees-C). Seasonal fluxes were calculated using air samples collected at 12-day intervals over 2 years (1991-1992). Gas-phase SIGMAPCB exhibited two concentration maxima: in May and in August. The monthly average homolog concentrations were modeled as the sum of two Lorentzian functions and applied to the two-film gas-exchange model. The largest volatilization fluxes were calculated for fall, when water temperatures were relatively warm (7-10-degrees-C) and vapor PCB concentrations were low (approximately 65 pg/m3). Vapor deposition to Lake Superior is indicated from late April through May, into 0-3-degrees-C water when vapor SIGMAPCB concentrations peak at approximately 200 pg/m3. The annual SIGMAPCB flux from Lake Superior for 1992 is 250 kg/yr.